Musculoskeletal joints include bones that move relative to one another. The motion of the bones is facilitated by lubricating synovial fluid that lies in a space (intra-articular space) between the moving bones in the joint. In living beings, the body produces the synovial fluid to ease the effort of moving the joints, and to reduce wear on the contact points of the moving bones.
Similar principles apply to the motion of living natural joints, artificial joints, or partially artificial joints. Artificial and partially artificial joints include one or more elements made of synthetic, artificial materials. An artificial lubricating fluid, or a wear-resistant smooth material can be used to make or coat the artificial portions of the artificial or partially artificial joints.
Testing and measurement of various physiological and mechanical properties of a musculoskeletal joint can be carried out by observing its response to predetermined conditions. For example, certain mechanical properties of a musculoskeletal joint can be tested by subjecting the joint to a force and observing the dynamic response of the joint to the applied force.
Experiments have been conducted on musculoskeletal joints to determine the coefficients of friction between parts of the joints that move relative to one another. Earlier works studied the lubrication of animal joints in the context of arthritis and rheumatism research using a pendulum connected to a portion of the joint. Some of these experiments involved attaching a pendulum to one portion of a musculoskeletal joint, fixing another portion of the joint, then observing the system's response when the pendulum is swung under the force of gravity. Properties of the joint could be deduced using the principles of mechanics to measure the frictional coefficients of the musculoskeletal joint for example.